Edge guard for abrasive polishing machines



1962 R. M. NELSON 3,048,950

EDGE GUARD FOR ABRASIVE POLISHING MACHINES Filed Aug. 4, 1960 2 Sheets-Sheet l INVENTOR. Poszer M N51. sow

Aug. 14, 1962 R. M. NELSON EDGE GUARD FOR ABRASIVE POLISHING MACHINES Filed Aug. 4, 1960 2 Sheets-Sheet 2 v 9 i w II "mm INVENTOR. ROBERT M. NELSOW A TTORA/EY Ute rates 3,048,950 EDGE GUARD FQR ABRASEVE POLISHIP I G MACHINES This invention relates to a guard for the transverse edge of a flat sheet of stock while it is passing through an abrasive polishing or sanding machine.

My invention is particularly adapted for use with an abrading device of the type which includes a rotatable drum which carries a plurality of abrasive coated flaps beneath which sheet material is passed by means of rollers or other suitable feeding means. The abrasive flaps tend to catch against the transverse edge of the sheet as it passes beneath the drum. In some cases, the edge of the sheet is actually lifted into the air by the flaps and the sheet is broken off, torn or otherwise seriously damaged.

It is an object of my invention to provide a simple but effective means for protecting the edges of such sheets as they pass beneath the abrasive drum. In essence, my invention contemplates a fiat guard plate of substantially the same thickness as the sheet. When the edge of each sheet approaches the drum, the guard plate is moved into abutting relationship with the edge of the sheet before it reaches the drum. The guard plate moves with the sheet until the sheet has safely passed beneath the drum and is then retracted until the edge of the next sheet approaches the abrasive drum.

In order to protect the guard plate and the bed of the machine from excessive abrasion by the abrasive drum, the abrasive drum may be lifted upwardly as the sheet leaves the abrasive drum and then brought down against as the next sheet moves toward the drum.

It is accordingly an object of my invention to provide an improved means for guarding the edges of work sheets against damage as they pass through an abrasive machine. It is a further object of my invention to provide a device of the type described which is simple in its structure and operation and which is adapted to be operated automatically so that its actions are automatically coordinated with the movement of the sheets through the machine.

Another object of my invention is to provide means for raising and lowering the abrasive drum in coordination with the movement of the work sheets, such means also being coordinated with the movements of the edge guard and also adapted to be operated automatically.

It is a further object of my invention to provide a device having all of the advantages and benefits of the structure set forth above and described in detail hereinafter. My invention also comprises such other objects, advantages and capabilities as will later more fully appear and which are inherently possessed by my invention.

While I have shown in the accompanying drawings a preferred embodiment of my invention, it should be understood that the same is susceptible of modification and change without departing from the spirit of my invention.

Referring to the drawings, FIG. 1 is a side elevational diagrammatic view of my machine in use with the parts shown in their positions when a work sheet is first inserted;

FIG. 2 is a similar view showing the sheet approaching the abrasive drum, which has been lowered;

FIG. 3 is a similar view showing the guard plate in tent 3,648,950 Patented Aug. 14, 1962 position protecting the trailing edge of the sheet as it passes beneath the abrasive drum;

FIG. 4 is a similar view showing the position of the parts after the sheet has passed by the abrasive drum, the drum has been raised and the guard plate is ready to be retracted;

FIG. 5 is an enlarged and more detailed side elevational view of the guard plate assembly;

FIG. 6 is a schematic diagram showing the electrical relationship of the switches and solenoid valves.

A preferred embodiment which has been selected to illustrate my invention comprises a structure in which the abrasive drum rotates in the same direction in which the sheet moves. The guard accordingly protects the trailing edge of the sheet. If the abrasive drum rotates in the opposite direction from the movement of the sheet, the guard would then be used to protect the leading edge of the sheet and its operation would be reversed.

The preferred embodiment of my invention comprises a fiat horizontal bed 10, along which a flat sheet 11 of material is adapted to be moved. The sheet 11 may be formed of wood, plywood, metal, plastic, rubber or any other material. The sheet 11 is moved along the bed Ill by means of a plurality of drive rollers 12.

An elongated cylindrical abrasive drum 13 which carries a plurality of radial abrasive leaves is rotatably mounted on a shaft 14 so that it extends transversely across the bed 10. The abrasive drum 13 .is rotated in the direction of movement of the sheet 11. The shaft 14 is mounted adjacent one end of a drum support 15, which is pivotally mounted adjacent its midportion on a vertical upright 16 which extends upwardly from the bed 10. The drum support 15 is adjustable vertically on the upright 16 to compensate for wear of the abrasive leaves, to adjust for vraiations in the thickness of the sheets 11 and to provide the desired amount of mushing" of the ends of the abrasive leaves against the sheet 11.

The opposite end of the drum support 15 is connected to the top of an upper arm 17. The bottom of the upper arm 17 is connected to the top of a lower arm 18. The bottom of the lower arm 18 is connected to a stationary bracket 19 which extends vertically downwardly from the bed 10.

A solenoid operated valve 20, which preferably comprises an air cylinder in which the valves are controlled by an electrical solenoid, is mounted on a stationary bracket 21 which extends downwardly beneath the bed 10. When its solenoid is energized, the valve 20 reciprocally moves a rod 22 outwardly and holds it in such position until the solenoid is de-energized. When the solenoid is de-energized, the valve 20 automatically retracts the rod 22. The rod 22 is connected to the arms 17 and 18 at their junction so that the valve 20 controls the flexing or straightening of the arms 17 and 18 to pivot the drum support 15 and move the abrasive drum 13 upwardly or downwardly with respect to the bed 10.

Extending vertically upwardly from the bed 10 is an upright 25. The upper end of a lever arm 26 is pivotally secured to the upper end of the upright 25. The lower end of the lever arm 26 carries a pivotally mounted link 27, the other end of which is pivotally secured to a bracket 23 which extends upwardly from a flat guard plate 28. The back edge of the guard plate 28 is normally held substantially above the bed 10 to permit the sheets 11 to pass beneath it. The link '27 permits the guard plate 23, upon pivotal movement of the lever arm 26, to be moved to a horizontal position overlying the bed 10.

The guard plate 28 is preferably formed of metal and normally corresponds substantially in thickness and width to the sheet 11. In the case of thicker sheets, the guard spas,

3 plate 28 may be of lesser thickness than the sheet 11, but it must be supported and disposed so that its upper surface is aligned with the upper surface of the sheet 11, to prevent the abrasive leaves from engaging the trailing edge of the sheet.

A substantially V-shaped frame 29 extends horizontally from the upright 25. Mounted on the frame 29 is a solenoid operated valve 24, which also preferably comprises an air cylinder in which the valves are controlled by an electrical solenoid. When its solenoid is energized, the valve 24 reciprocally moves an arm 30 outwardly. The forward end of the arm 30 is connected to the lever arm 26 adjacent the midportion thereof.

An angular hold-down bar 34 is fixedly mounted beneath the abrasive drum 13 to overcome warpage and hold the leading edge of the sheet 11 down against the bed as it approaches the abrasive drum 13. The portion of the hold-down bar 34 from which the sheet 11 approaches is angled upwardly so that it will contact the leading edge of the sheet 11 even if the leading edge is disposed substantially above the bed 16 due to warpage. The remaining portion of the hold-down bar 34 is directed horizontally and acts to hold the sheet 11 flat against the bed 10 during the time when its trailing edge is engaged by the guard plate 28. The engagement between the leading edge of the guard plate 28 and the trailing edge of the sheet 11 takes place beneath the horizontal portion of the hold-down bar 34 in order to assure alignment of the guard plate 28 and sheet 11. Thereafter, the guard plate 28 is held in engagement with the sheet 11 by the solenoid operated valve 24.

A switch 35 is mounted adjacent the hold-down bar 34 beneath the bed lit. The switch 35 has an operating arm which protrudes upwardly through an opening in the bed 10. The operating arm of the switch 35 is disposed to one side of the sheet 11 so that it is not depressed by the sheet 11. The guard plate 28 is slightly wider than the sheet 11 so that it engages and depresses the operating arm of the switch 35 if a sheet lll is not disposed beneath the guard plate 28. When a sheet 11 passes beneath the guard plate 28, it moves the guard plate 23 upwardly so that the operating arm of the switch 35 is no longer depressed.

Another switch 36 is disposed beneath the bed 10 on the opposite side of the abrasive drum 13 from the switch 35. The switch 36 likewise has an operating arm which extends vertically upwardly through an opening in the bed lit The operating arm of the switch 36 is adapted to be depressed by the sheet 11 as it moves along the bed 10 past the switch 36.

A substantially L-shaped stock stop 37 is mounted beneath the bed 10 ahead of the first set of rollers 12. The upper end of the stock stop 37 protrudes through an opening in the bed 18*. A solenoid operated valve 38, which preferably comprises an air cylinder in which the valves are controlled by an electrical solenoid, is connected to the lower end of the stock stop 37. When its solenoid is energized, the valve 38 retracts the stock stop 37 to permit a sheet 11 to pass. The stock stop 37 prevents a new sheet 11 from approaching the abrasive drum 13 until the guard plate 23 has completed its function of protecting the trailing edge of the preceding sheet 11 and has been retracted to its normal position.

The electrical relationship of the solenoid operated valves 20. 24 and 33 and the switches 35 and 36 is shown in FIG. 6 of the drawings. FIG. 6 shows the switches as they are disposed when the device is ready for use. The parts are then disposed in the positions shown in FIG. 1 of the drawings. The stock stop 37 is depressed due to the energization of its solenoid operated valve 38 through the normally closed bottom half of the switch 36. The sheet 11 is accordingly permitted to pass the stock stop 37 and approach the abrasive drum 13.

When the sheet 11 passes beneath the guard plate 28, the guard plate 28 is raised, permitting the operating arm of the switch 35 to move upwardly so that the switch 35 changes its position. This change closes a circuit through the normally open bottom half of the switch 35 to a time delay 33. After a short time interval the solenoid operated valve is energized, causing its rod 22 to be moved outwardly, straightening the arms 17 and 18 to pivot the drum support 15 and bring the abrasive drum 13 downwardly into position for engaging the surface of the sheet 11 when it reaches the drum 13. The parts then assume the positions shown in FIG. 2 of the drawings.

The switch 36 is operated as the sheet 11 depresses its operating arm, opening its normally closed bottom half to de-energize the solenoid operated valve 38 and permit the stock stop 37 to move into a position blocking another sheet 11 from moving past it toward the abrasive drum 13. The closing of the top half of the switch 36 has no effect because it is in series with the open top half of the switch 35. The relationship of the parts remains the same while the sheet 11 passes beneath the abrasive drum 13.

When the trailing edge of the sheet 11 passes beneath the leading edge of the guard plate 28, the guard plate 28 drops down to depress the operating arm of the switch 35 and change its electrical positions back to normal. Since the operating arm of the switch 36 is still depressed by the sheet 11, an electrical circuit is completed through the top halves of the switches 35 and 36 to the solenoid operated valve 24, causing its arm to be moved forwardly to pivot the lever arm 26 in a forward direction. This moves the leading edge of the guard plate 28 into abutting relationship with the trailing edge of the sheet 11.

Simultaneously, a circuit is opened through the bottom half of the switch to the time delay 33. After a short interval the solenoid operated Valve 20 is de-energized, the rod 22 retracted and the abrasive drum 13 raised to minimize its abrasive action against the guard plate 28 or bed 10.

When the trailing edge of the sheet 11 passes the switch 36, the switch 36 moves back to its normal position. This opens the circuit to the solenoid operated valve 24, retracting the uard plate 28. It also closes a circuit to the solenoid operated valve 38, retracting the stock stop 37 to permit another sheet 11 to move toward the abrasive drum 13. The cycle has then been completed and the parts are back to the positions shown in FIG. 1 of the drawings.

The time delay 33 serves the purpose of preventing the abrasive drum 13 from moving down and unduly abrading the bed 16 before the leading edge of the sheet 11 reaches the drum 13. It also delays the raising of the drum 13 until the trailing edge of the sheet 11 has moved past the drum 13. The timing of these movements is not critical, as long as the drum 13 engages the entire surface of the sheet 11. Minor discrepancies in the timing beyond this point merely result in variations in the amount of abrading action of the drum 13 against the bed 10 and guard plate 28.

I claim:

1. In an abrasive drum assembly having a bed across which a sheet is adapted to be moved, an abrasive drum extending substantially transversely across said bed and means for rotating said abrasive drum with respect to said sheet, a guard plate having a width substantially corresponding to the width of said sheet, and means for moving said guard plate into substantially abutting relationship with one of the transverse edges of said sheet before it reaches said abrasive drum to protect said edge of said sheet from being damaged by said abrasive drum, said means for moving said guard plate comprising a solenoid controlled valve and means connecting said solenoid controlled valve to said guard plate.

2. The structure described in claim 1 and an electric switch operable upon the approach of the edge of said sheet toward said abrasive drum to operate said solenoid valve and move said guard plate into position behind said sheet.

3. In an abrasive drum assembly having a bed across which a sheet is adapted to be moved, an abrasive drum extending substantially transversely across said bed and means for rotating said abrasive drum in the direction of movement of said sheet, a guard plate having a width substantially corresponding to the width of said sheet, said guard plate being disposed toward the side of said abrasive drum from which said sheet is moved and being mounted above said bed so that said sheet can be moved beneath said guard plate toward said abrasive drum, and means for moving said guard plate into substantially abutting relationship with the trailing edge of said sheet before it reaches said abrasive drum to protect the trailing edge of said sheet from being damaged by said abrasive drum.

4. In an abrasive drum assembly having a bed across which a sheet is adapted to be moved, an abrasive drum extending substantially transversely across said bed and means for rotating said abrasive drum in the direction of movement of said sheet, a flat guard plate having a width and thickness substantially corresponding to those of said sheet, a pivotally mounted lever arm disposed adjacent to said abrasive drum, and linkage connecting said guard plate to said lever arm to permit said guard plate to be moved into a substantially flat horizontal position overlying the top of said bed, and means for pivoting said lever arm to move said guard plate into such position in substantially abutting relationship with the trailing edge of said sheet before it reaches said abrasive drum to protect the trailing edge of said sheet from being damaged by said abrasive drum.

5. In an abrasive drum assembly having a fiat horizontal bed across which a sheet of material is adapted to be moved, an abrasive drum extending substantially transversely across said bed, said abrasive drum being carried by a drum support, said drum support being mounted for pivotal movement to move said abrasive drum vertically with respect to said bed, means for rotating said abrasive drum in the direction of movement of said sheet, a lever arm disposed adjacent to said abrasive drum, the upper end of said lever arm being pivotally secured at a point above said bed, a guard plate con nected to said lever arm, said guard plate having a width substantially corresponding to that of said sheet, said guard plate being mounted above said bed so that said sheet can pass beneath said guard plate toward said abrasive drum, means for pivoting said lever arm to move said guard plate into substantially abutting relationship with the trailing edge of said sheet before it reaches said abrasive drum to protect the trailing edge of said sheet from being damaged by said abrasive drum, and means for pivoting said drum support to move said abrasive drum upwardly away from said bed after the trailing edge of said sheet has passed the center of said abrasive drum.

6. The structure described in claim 5, said means for ivoting said drum support comprising a solenoid valve, a reciprocating rod operable by said solenoid valve, said rod being connected to said drum support so that the pivoting of said drum support is controlled by said solenoid valve and an electric switch operable upon the movement of the trailing edge of said sheet past the center of said abrasive drum to operate said solenoid valve and move said abrasive drum away from said bed.

7. In an abrasive drum assembly having a flat horizontal bed across which a sheet of material is adapted to be moved, an abrasive drum extending substantially transversely across said bed, said abrasive drum being carried by a drum support, said drum support being mounted for pivotal movement to move said abrasive drum vertically with respect to said bed, a first solenoid valve mounted adjacent said drum support, a reciprocating rod operable by said first solenoid valve, said rod being connected to said drum support so that the pivoting of said drum support is controlled by said first solenoid valve, means for rotating said abrasive drum in the direction of movement of said sheet, a lever arm disposed adjacent to said abrasive drum, the upper end of said lever arm being pivotally secured at a point above said bed, a second solenoid valve mounted adjacent to said lever arm, a reciprocating rod operable by said solenoid valve, said rod being connected to said lever arm so that the pivoting of said lever arm is controlled by said second solenoid valve, a guard plate connected to said lever arm, said guard plate having a width and thickness substantially corresponding to those of said sheet, the trailing edge of said guard plate normally being disposed above said bed so that said sheet can pass beneath said guard plate toward said abrasive drum, an electric switch operable upon the approach of the trailing edge of said sheet toward said abrasive drum to ope-rate said second solenoid valve and move said guard plate into substantially abutting relationship with the trailing edge of said sheet before it reaches said abrasive drum to protect the trailing edge of said sheet from being damaged by said abrasive drum, and a second electric switch operable upon the movement of the trailing edge of said sheet past the center of said abrasive drum to operate said first solenoid valve and pivot said drum support to move said abrasive drum upwardly away from said bed.

8. The structure described in claim 7 and a stop stock disposed along said bed on the side of said abrasive drum from which said sheet is moved, said stop stock being movable into said bed to block the movement of another sheet toward said drum, and electrically operable means for controlling the movement of said stop stock to block the movement of another sheet toward said abrasive drum whenever said guard plate is in a postion other than its normal position above said bed.

9. In an abrasive assembly having a bed across which a sheet is adapted to be moved, an abrasive member extending substantially transversely across said bed, means for moving said abrasive member with respect to said sheet during the movement of the sheet to provide an abrading action on the sheet, a guard plate disposed transversely relative to the sheet at a position displaced from the plane of the sheet and movable toward the sheet, and means responsive to the movement of the sheet into a particular relationship with respect to the abrading member for moving said guard plate into substantially abutting relationship with said sheet before the trailing end of the sheet reaches said abrasive member to protect the trailing edge of said sheet from being damaged by said abrasive member.

10. In an abrasive assembly having a bed across which a sheet is adapted to be moved, an abrasive member extending substantially transversely across said bed, means operately coupled to said abrasive member for obtaining an abrading action by said abrasive member on said sheet during the movement of said sheet past said abrasive member, a guard plate disposed transversely relative to said sheet at a position in front of said abrasive member in the direction of movement of said sheet, the guard plate being movable in a direction transverse to the movement of the sheet to retain the sheet on the bed and being movable in the direction of the sheet to guide the sheet past the abrasive member, and means for moving said guard plate in the transverse direction and in the direction of movement of the sheet to protect said sheet from being damaged by said abrasive member.

11. In an abrasive assembly having a bed across which a sheet is adapted to be moved, an abrasive member extending substantially transversely across said bed, means for obtaining a movement of said abrasive member relative to said sheet in the direction of movement of said sheet to engage said sheet during the movement of the sheet and provide an abrading action on the sheet, a guard plate extending in the transverse direction and normally displaced in displaced relationship to the sheet on the bed and movable into a position to engage said sheet, said guard plate being disposed toward the side aoaspso of said abrasive member from which said sheet is moved and being mounted above said bed so that said sheet can be moved beneath said guard plate toward said abrasive member, and means for moving said guard plate into substantially abutting relationship with the trailing edge of said sheet before it reaches said abrasive member to protect the trailing edge of said sheet from being damaged by said abrasive member.

12. In an abrasive assembly having a bed across which a sheet is adapated to be moved, an abrasive member extending substantially transversely across said bed, means for moving said abrasive member in the direction of movement of said sheet, a guard plate disposed in the transverse direction and movable to a position to engage the sheet during the movement of the sheet, a pivotally mounted lever arm, and linkage connecting said guard plate to said lever arm to obtain a movement of said guard plate into a position overlying the top of said bed and abutting the sheet upon a pivotal movement of said lever arm, and means for pivoting said lever arm to move said guard plate into the overlying position in substantially abutting relationship with said sheet before the trailing edge of said sheet reaches said abrasive member to protect said sheet from being damaged by said abrasive member.

13. In an abrasive assembly having a bed across which a sheet is adapted to be moved, an abrasive member extending in a substantially transverse direction across the bed, means for obtaining a movement of the abrasive member in a direction for providing an abrasive action on the sheet during the movement of the sheet along the bed, guard means disposed relative to the sheet and the bed to provide for a movement of the sheet between the guard means and the bed in contiguity with the guard means and disposed in front of the abrasive member in the direction of movement of the sheet, the guard means being movable from a first position displaced from the bed to a second position toward the bed upon the move ment of each sheet past the guard means to obtain an engagement of the sheet by the guard means, and means responsive to the movement of the guard means to the second position for moving the guard means in the direction of movement of the sheet and in accordance with the movements of the sheet in that direction to maintain the sheet on the bed and prevent the sheet from becoming damaged by the abrasive member.

14. The combination set forth in claim 13 in which means are responsive to the movement of each sheet on the bed past the abrasive member to prevent the next sheet from being moved past the abrasive member during the movement of the first sheet past the abrasive member.

15. In an abrasive assembly having a bed across which a sheet is adapted to be moved, an abrasive member extending in a substantially transverse direction across the bed, the abrasive member being normally disposed in displaced relationship to the bed, means operatively coupled to the abrasive member for obtaining a movement of the abrasive member to the bed upon a movement of the sheet along the bed to a position for receiving an abrading action by the abrasive member, means operatively coupled to the abrasive member for providing a movement of the abrasive member in a direction for providing an abrasive action on the sheet during the movement of the sheet along the bed, guard means normally disposed in displaced relationship from the bed and movable toward the bed upon a movement of the sheet past the abrasive member to prevent the sheet from being damaged by the abrasive member, and means operatively coupled to the guard means and responsive to the movement of the sheet past the abrasive member for obtaining a movement of the guard means toward the abrasive member to prevent the sheet from being damaged by the abrasive member.

16. The abrasive assembly set forth in claim 15 in which the guard means is disposed in front of the abrasive member in the direction of movement of the sheet to prevent the rear end of the sheet from being damaged by the abrasive member and in which the guard means is moved toward the bed after the movement of the front end of the sheet past the guard means.

17. In an abrasive assembly having a bed across which a sheet is adapted to be moved, an abrading member extending in a substantially transverse direction across the bed, means for moving the abrading member in a direction to abrade the sheet during the movement of the sheet along the bed, guard means normally disposed in a first position in spaced relationship to the bed and responsive to the movement of the sheet along the bed to the abrading member to become disposed in a second position for maintaining the sheet on the bed, switching means responsive to the movement of the guard means from the first position to the second position to become actuated, and means responsive to the actuation of the switching means for driving the guard means in the second position of the guard means and in the direction of movement of the sheet in accordance with the movement of the sheet along the bed for maintaining the sheet on the bed during the movement of the sheet to prevent the sheet from being damaged by the abrading member.

18. In an abrasive assembly having a bed across which a sheet is adapted to be moved, an abrasive member extending in a substantially transverse direction across said bed, means for moving the abrasive member relative to the bed during the movement of the sheet across the bed to provide an abrading action on the sheet, guard means disposed transversely across the bed in spaced relationship to the bed and movable to a position for engaging the sheet during the movement of the sheet along the bed, means responsive to the movement of the sheet to the position for engagement by the abrasive member for ac tuating the guard means for movement, and means operatively coupled to the guard means for obtaining a movement of the guard means in the direction of movement of the sheet along the bed and in accordance with the movement of the sheet along the bed to prevent the sheet from being damaged by the abrasive member.

19. In an abrasive assembly having a bed across which a sheet is adapted to be moved, an abrasive member extending in a substantially transverse direction across the bed and disposed at a position displaced from the bed, means operatively coupled to the abrasive member for obtaining a movement of the abrasive member in a direction for providing an abrasive action on the sheet during the movement of the sheet along the bed, means operative- 1y coupled to the abrasive member for obtaining a movement of the abrasive member toward the bed to obtain an engagement between the abrasive member and the sheet upon a movement of the sheet to a position for receiving an abrading action, guard means disposed relative to the sheet during the movement of the sheet past the abrasive member for preventing the edges of the sheet from being damaged by the abrasive member, the guard F means being movable in the direction of movement of the sheet in accordance with the movements of the sheet to prevent the edges of the sheet from being damaged by the abrasive member during the movement of the sheet past the abrasive member, and means operatively coupled to the guard means for providing a movement of the guard means in the direction of movement of the sheet during the movement of the sheet past the abrasive member.

20. In an abrasive assembly having a bed across which a sheet is adapted to be moved, an abrasive member extending in a substantially transverse direction across the bed, means operatively coupled to the abrasive member for obtaining a movement of the abrasive member in a direction for providing an abrasive action on the sheet, guard means disposed in front of the abrasive member in the direction of movement of the sheet and normally displaced from the bed and movable toward the bed to a particular position for engaging the rear end of the sheet to prevent the rear end of the sheet from being damaged by the abrasive member, and means operatively coupled to the guard means and responsive to the movement of a it) the sheet to the abrasive member for obtaining a moveuntil after the movement of the first sheet. past the abrasive merit of the guard means to the particular position to premember. vent the rear end of the sheet from being damaged by the abrasive member- References Cited in the fil of this e patent 21. The abrasive assembly set forth in claim 20 m 5 which means are responsive to the movement of the sheet UNITED STATES PATENTS past the abrasive member to prevent any additional sheet 2,748,417 Heim June 5, 1956 from being moved along the bed to the abrasive member 

